This invention relates to an internal combustion engine operable on two kinds of fuel provided in amounts which are controlled in accordance with engine operating conditions.
In recent years, the reduction of petroleum consumption has become of prime concern. Consequently, it is desired to save gasoline by using alcohol, such as ethanol or methanol, as a fuel on which automobile engines operate. Attempts have been made to operate automobile engines on gasoline fuel mixed with alcohol, but such attempts have failed to make the efficient use of the advantages of alcohol. They did not provide the desired energy conversion efficiency although they did achieve a saving in petroleum (gasoline) consumption.
Alcohol has a high octane number of 100 or more and a high degree of latent heat, that is, the heat required for vaporization. Ethanol has a latent heat three times as high as gasoline and methanol has a latent heat four times as high as gasoline. One of the great advantages of Alcohol fuel as opposed to gasoline fuel is the capacity of providing a high compression ratio owing to its higher octane number and owing to the cooling effect of its higher latent heat. For example, an alcohol fuel containing 100 percent methanol can increase the compression ratio as much as 15 to 1.
With gasoline fuel, alternatively, the compression ratio is limited to below 8.5 to 1 or 9 to 1 owing to its low octane number due to anti-knock considerations. It can be seen that with a gasoline fuel into which alcohol is incorporated, the compression ratio should be determined based upon the octane number of the gasoline fuel and therefore the heat energy content of the alcohol is utilized insufficiently.
Alcohol has a low boiling point. The boiling point of methanol is at 64.degree. C. and that of ethanol is at 78.degree. C. The use of alcohol fuel for gasoline engines tends to spoil their starting and warming up capabilities.
In Brazil, alcohol fueled motor vehicles are marketed. These vehicles are equipped with a mechanism which controls the fuel supplied to the engine in such a manner that the engine operates on gasoline fuel during start up operation and on alcohol fuel after starting.
Another kind of fuel control system for alcohol fueled engine is also proposed in which the engine operates on gasoline fuel during start up and warming up operations and on alcohol fuel during normal operation, that is, operation after the warming up has completed.
In the above two cases, the change of the kind of fuel supplied to the engine from gasoline to alcohol depends on the three operating conditions of the engine, i.e. starting up, warming up and normal operating conditions. Since the fuel change is performed while the engine load is not drastically changing, a relatively simple and low response control system for changing fuel is employed.
However, when fuel change is required in accordance with the engine load, the control device must perform smooth and quick response fuel change, since the engine load is changed very frequently during normal engine operating condition.
The present invention, therefore, provides an internal combustion engine operable on two kinds of fuel properly selected with a high response to engine load conditions and smoothly operable without any output torque variations, at the chance of selection of fuel from one kind to the other. The present invention can make efficient use of the advantages of both kinds of fuel and save gasoline without any reduction in engine drivability.